Problem

The measurements started when the gearbox was in healthy condition and continued throughout the period during which bearing and gear condition deteriorated, and continued after the gearbox had been repaired.

Solution

The following measurement techniques were used on all sensors:

• SPM HD TSA – SPMHD and Time Synchronous Averaging (TSA)
• HD ENV3 – HD ENV filter 500 – 10,000 Hz
• HD ENV3 TSA – HD ENV filter 500 – 10,000 Hz and Time Synchronous Averaging
• HD ENV4 TSA – HD ENV filter 5 000 – 40,000 Hz and Time Synchronous Averaging
• VEL TSA – Velocity and Time Synchronous Averaging

The reason for using mainly TSA measurements is that we were focusing on the gears, while the explanation for the relatively many measurement assignments is that we wanted to show how the different assignments respond if a gear problem were to occur.

To be able to do TSA measurements on intermediate shafts, we need to use the Pseudo Tach function. A normal TSA requires a trigger pulse from the very same shaft where the gears are mounted. By using Pseudo Tach, an artificial and VERY accurate trigger pulse is used to trigger measurements.

If, say 200 averages, is used and the artificial signal is not accurate enough, the fault will increase for every revolution and the result will be meaningless. By using HD Technology also for the Pseudo Tach, accuracy is ensured.

Conclusion

The root cause of this gearbox damage was that water had accidentally entered the gearbox during cleaning of the shaft. The water adversely affected bearing lubrication, hence damaging the bearings. Consequently, the gears did not work properly and were also damaged.

It has been an instructive project where we have successfully detected gear damage in the gearbox. Among other things, the measurements from this and other gearboxes have resulted in recommendations regarding the most suitable measurement techniques for different gearbox problems.

The results in this case study show that HD ENV is ideal for detecting gear problems such as gear mesh, while SPM HD works very well for monitoring bearing and lubrication condition. For successful gearbox monitoring, we recommend using one sensor per shaft, with the following measurement assignments set up on all sensors:

Bearing condition assignments

• VEL
• HD ENV, Filter 3
• HD ENV, Filter 4, or SPM HD Gear condition assignments
• VEL TSA
• HD ENV, Filter 3 with TSA

Pseudo Tach will automatically be activated when using an RPM factor. To ensure the factor is accurate enough, enter the number of teeth on the gearwheels in the RPM factor settings.